Field of the Invention
The invention relates to a method and a device for determining the latency or length of an optical path, especially an optical fiber, of a fiber-optical transmission link. Further, the invention relates to an optical transmission link comprising two optical nodes that are connected via an optical path, wherein the optical nodes comprise a respective device for determining the length of the optical path therebetween. Finally, the invention relates to a method for routing signals in an optical network and a network implementing this method, wherein the latencies between two network nodes are used in order to determine a route which fulfills a given latency constraint.
Description of Related Art
The signal travel time is one of the major routine parameters in case of dynamically switched optical networks. Knowing the “fastest” path through a network improves the signal latency, i.e. the time interval between transmitting a signal at a first point or node of the networks and receiving the signal at a second point or network nodes including the signal delay due to signal switching (and other signal processing operations) and the mere signal propagation time, which, in an optical network, is mainly influenced by the length of the respective total optical path. The total optical path may of course consist of a plurality of partial optical paths or links. Although the invention is applicable to any type of optical path, in most practical applications the optical path or partial optical path will consist of one or more optical fibers only.
Usually, the transmission fiber length is estimated after finishing the fiber cable installation or by distance calculations based on a geographical map.
It is, however, also possible to measure the fiber length after having finished the fiber cable installation. Also, the fiber length may be measured, if necessary, after the optical network installation has been finished or the structure thereof has been changed. Measurement of the length of an optical path is usually carried out by using appropriate measurement equipment, which, in most cases, uses a method based on optical pulse delay measurement. Such a measurement device, for example an optical time domain reflectometer (OTDR), is typically not integrated into a signal transmission equipment due to additional effort and costs related therewith.
Other measurement methods require a sophisticated transmission protocol that is used for the signal transmission between the two end points of the respective transmission link. In many cases, such measurement methods require synchronized clocks on both ends of the transmission link as the propagation delay of pulses is recorded.
Signal latency measurements are also implemented in transmission protocol standards like the ITU-T standard, which defines an optical transport network (OTN) as a set of optical network elements (ONE) connected by optical fiber links.
It is thus an object of the present invention to provide a method for determining the latency or length of an optical path, especially an optical fiber, of a fiber-optical transmission link, which can be implemented easily and at low costs and which does not require synchronized clocks at both ends of the transmission link. It is a further object of the invention to provide a device realizing the method according to the invention and an optical transmission link comprising such a device. Finally, it is an object of the invention to provide a method for routing signals in an optical network and a network implementing this method, wherein a given constraint for a maximum latency of a selected route between two network nodes is fulfilled.